This invention relates to a process for the amidation of vinyl chloride (VCl), and the discovery of a surprising difference in the rate of this amidation compared with that of structurally similar vinylic chlorides when a zero valent monodentate palladium catalyst is used under certain conditions which unexpectedly also provide the catalyst with very long life. Amidation refers to the known reaction of an amine with carbon monoxide, under elevated temperature and pressure, to produce an amide. The commercial promise of amidation reactions with known catalysts has never materialized. Among the main reasons is the discouragingly low reaction rates with all but the organo-bromides and organoiodides which are of little commercial significance. Thus, over the years, those skilled in the art had a well-established experimental basis for eschewing further investigation of amidation applied to chloroalkenes and chloroarenes.
The particular promise of the amidation of VCl is based on the unexpectedly high rate of the rate-controlling oxidative addition step which leads to the nearly quantitative formation of the Michael adduct (formed by a Michael addition) of the amine reactant with the acrylamide or N-substituted acrylamide produced (both referred to hereinafter as "acrylamides" for brevity). This Michael adduct formation occurs only with VCl and no other chloroalkene, with unexpectedly high selectivity while maintaining excellent catalyst performance, combined with extended catalyst life, though the catalyst is a known palladium catalyst. This process affords a practical route to the manufacture of acrylamide, N-phenylmethacrylamide, and N,N-dimethylacrylamide, each of which is useful in a wide array of polymers. Dimethylamine in particular provides greatly enhanced catalyst stability, because of its ability to overcome the causes of deactivation which I have identified.
More specifically this invention relates to the use of known monodentate tertiary-phosphine complexes of palladium(0), such as tetrakis(triphenylphosphine)palladium, per se, or generated in situ, which are far more effective than other palladium catalysts, or Group VIII catalysts, in the amidation of VCl with ammonia and CO; and with certain primary and secondary amines, all of which form the Michael adduct. The Michael adduct, derived from the addition of amine to the acrylamide produced is then conveniently and efficiently converted to the desired acrylamide.
This amidation process involves a series of reactions in which the key step is the oxidative addition of Pd(0) to the organohalide. It is well-known that oxidative addition occurs far more rapidly with bromides and iodides than with chlorides. In the art, the only example of a monochloroalkene undergoing amidation to an alpha-beta unsaturated amide is that of 2-chloropropene disclosed in U.S. Pat. Nos. 3,988,358 and 4,128,554 to Heck. A carbonylation reaction, only superficially analogous to the foregoing Heck amidation, and presumably therefore, not cited in Heck's '554 patent, is the process disclosed in U.S. Pat. No. 3,991,101 to Knifton. Knifton provides a specific illustration of carbonylation of VCl itself. In part A of Example 1, he uses Pd(PPh.sub.3)Cl.sub.2, the same Pd(II) catalyst used by Heck, together with stannous chloride co-catalyst for the carbonylation in methanol to produce methyl acrylate in 83% yield. But in part B, the same Pd(PPh.sub.3)C.sub.2 catalyst, but with no stannous chloride, result in a different product. He found that "the major reaction is CO addition to the carbon-carbon double bond and that the major portion by far is methyl alpha-chloropropionate (selectivity %-74), with approximately one quarter (1/4) as much of the desired methyl acrylate being formed (selectivity %-18) plus a significant quantity of dimethyl alpha-methyl malonate (selectivity %-8)". In other words, the main product is the saturated ester resulting from addition to the double bond rather than substitution of chlorine.
Generally, carbonylation reactions of halogen-substituted unsaturated compounds, are well known. But the reactions are also well known for their differences in rates and selectivity. These differences can depend upon whether the compound is an allylic, aryl or a vinyl halide, and as illustrated by Heck, also on the type of halide, e.g. bromides and iodides which do not require the forcing conditions of vinylic chlorides because the latter are so unreactive.
The surprisingly high rates obtained with aryl and vinylic halides as compared with analogous palladium-catalyzed reactions with allylic halides was the basis of the Heck '358 and '554 inventions. In the parent '358 reference he states that his object is "to produce carboxylic esters and amides in good yields under mild conditions from organic halides other than allylic halides, including those organic halides that have been considered to be unreactive as compared to allylic halides." (see col 1, lines 21-26, emphasis supplied).
Consistent with this objective, Heck's investigation of the amidation reaction was generally limited to atmospheric pressure and a temperature in the range from 60.degree.-100.degree. C. (see first line of "Results and Discussion" of the article titled "Palladium-Catalyzed Amidation of Aryl, Heterocyclic, and Vinylic Halides" by A. Schoenberg and R.F. Heck, 39 Jour. Org. Chem., p 3327 (1974)) and examples in the '358 and '554 patents. However, the rea of 2-chloropropene with CO and aniline in the presence of a stoichiometric quantity of tri-n-butylamine was at 135.degree. C. to 140.degree. C. and pressure of about 800 psig for periods of 12 to 24 hr (see example 33 of '358 patent, example 13 of the '554 patent, and experimental section of the article, pg 3330). With further reference to rates, note that the Heck disclosure is related to any non-allylic halide, specifically covering chlorides, bromides and iodides without suggesting that there may be any distinguishing characteristics in the reactivity of one halide over the other, that would be critically important to their commercial importance.
That there should be a notable difference in the rates obtained with various non-allylic halides is expected. This is especially true when comparing bromides and iodides with chlorides and fluorides. The difference in rates between chlorides on the one hand, and bromides and iodides on the other, is confirmed by comparing rough estimates of rates calculated from Heck's examples. These are estimates because the Heck experiments with 2-chloro-propene were directed to determining yields, not rates--the stated object to the contrary. His experiments were carried out to unspecified high conversions of aniline, the limiting reagent. Therefore, when comparing rates of product formation among the bromides, iodides, and 2-chloropropene, one must assume that the reported reaction times correspond to the time required to reach roughly the same aniline conversion. Such a comparison shows a substantially slower rate for 2-chloropropene despite more vigorous conditions. To avoid this ambiguity, I have measured and compared rates of all three chloropropenes with VCl under the same conditions.
In my experiments I have measured the continuous disappearance of amine with time and expressed this as "turnover rate". The turnover rate used herein is defined as:
(moles of reactant converted) divided by (moles of catalyst) for each hour of reaction time. This form of measurement of rate is routinely used in the art (see for example "Catalysis and Inhibition of Chemical Reactions", by P.G. Ashmore, pp 9, Butterworths, London, 1963).
In the illustrative examples I have presented for VCl amidation, I have reported the rate of disappearance of ammonia used to convert VCl simply because this rate can be measured continuously by gas chromatography. However, when I compared the amidation of VCl with the three chloropropenes, the latter reactions were so slow that the rate of disappearance of ammonia could not be measured accurately. In this case, it is far more accurate to measure the formation of chloride ion as NH.sub.4 Cl by titration, which I did, and reported the relative rates (see example 8 herein).
The rates in the Heck references may be compared with those I obtained if they are converted to "turnover rate"with the appropriate assumptions. Despite the high cost of catalyst and the economics of rates, Heck's reports of his results focused more on yield than on catalyst longevity or rates.
It is well recognized in the art that the oxidation addition step which activates the carbon-halogen bond in this process is very slow for vinyl or aromatic carbon-X bonds when X is Cl, compared with Br or I. Therefore bromides and iodides are usually used when this activation, step is required in a chemical process. Thus, lactams are prepared from bromoalkenes and iodoalkenes having secondary substituents. See M. Mori, et al Jour. Org. Chem. 48 p 4058 (1983). Benzolactams are prepared from o-bromoamino-alkylbenzene by photochemical carbonylation under phase transfer conditions using cobalt carbonyl as the catalyst. See J. Brunet, et al Jour. Org. Chem. 48 p 1166 (1983). All but one of the many examples in the Heck references relate to bromides and iodides. The resistance of chloroalkyls, monochloroalkenes and chloroarenes toward oxidation addition with zero-valent Ni, Pd and Pt complexes is well documented. See P. Fitton, et al Jour. Organometallic Chem. 28 p 287 (1971); J.T. Colman, et al Principles and Applications of Organotransition Metal Chemistry p 185 (1980); and, R.F. Heck, et al Catalysis in Organic Synthesis 7th ed. p 195-218, inter alia.
The presence of the methyl group in 2-chloropropene is the reason for the low reaction rate. Moreover, it suppresses formation of the Michael adduct derived from addition of amine with the acrylamide produced. Reactivity is greatly enhanced when the monochloroalkene is unsubstituted, that is, VCl. Heck did not discover that VCl would give the high reaction rate and form the Michael adduct. Though he includes VBr in his broad disclosure of useful reactants, there is nothing in his disclosure to suggest that VBr may behave differently from the host of other compounds since there is no mention anywhere, of the Michael adduct. Not having run VBr or VCl, he could not know that the formation of the Michael adduct depends upon the unique unsubstituted character of VCl or VBr, and that it has an important influence on catalyst stability. The Michael adduct is critical to catalyst stability because it suppresses formation of the phosphonium chloride which removes stabilizing ligand from the metal in the complex, thus causing the metal to separate.
Moreover, Heck added a stoichiometric amount of tri-n-butylamine to remove HCl when a weakly basic amine is amidated. The presence of a basic tertiary amine in a molar equivalent amount compared to the weakly basic amine, or in excess of that amount, was claimed to be a necessary condition to enable one to carry out the Heck invention in most instances. It is unnecessary in VCl.
Still another difference that I have observed relates to the role of the amine as the reducing agent for the catalyst complex. In every Heck example, the catalyst used was a Pd.sup.2 +complex (e.g. Pd(PPh.sub.3).sub.2)Cl.sub.2) which was undoubtedly reduced to the active Pd(0) complex in situ. I have found that the amine is the principal reducing agent for this process, at least with VCl, and that a suitable amine is one that contains alkyl substituents having alpha hydrogen atoms. Thus, while either Pd(PPh.sub.3).sub.2)Cl.sub.2 +2PPh.sub.3 or Pd(PPh).sub.4 are equivalent catalysts when dimethylamine is the amine reactant, they are not equivalent when using ammonia. Pd(PPh.sub.3).sub.2 Cl.sub.2 is essentially inactive in VCl amidation with ammonia as shown in example 7 herein; therefore Heck's disclosure relating to the amidation of VCl with the catalyst he used, is not an enabling disclosure. The choice of amine can be an important factor in obtaining catalyst longevity, depending upon the extent to which Pd(PR.sub.3).sub.2 Cl.sub.2, defined herebelow, forms during amidation.
Had Heck run the reaction with VCl instead of 2-chloropropenes, he would have observed its high reactivity, the formation of the Michael adduct, its importance to catalyst stability, and reported the results.